Power over LAN™ is a new technology that enables DC power to be supplied to Ethernet data terminals over ordinary Category 5 cabling. This technology enables the terminals to receive their operating power over the same Ethernet local area network (LAN) that they use for data communication. It thus eliminates the need to connect each terminal to an AC power socket, and to provide each terminal with its own AC/DC power converter. Further aspects of this technology are described in PowerDsine Application Note 115, entitled “Power over LAN™: Building Power Ready Devices” (PowerDsine Ltd., Hod Hasharon, Israel), which is incorporated herein by reference. The LAN MAN Standards Committee of the IEEE Computer Society is developing specifications for Power over LAN systems, as described in IEEE Draft P802.3af/D3.0, entitled “Data Terminal Equipment (DTE) Power via Media Dependent Interface (MDI)” (IEEE Standards Department, Piscataway, N.J., 2001), which is also incorporated herein by reference.
A Power over LAN system comprises an Ethernet switch and a power hub, which supplies DC power, along with a number of terminals, which communicate via the switch and draw power from the hub. The system is typically connected in a star topology, with each terminal linked by a dedicated cable to the switch and hub. DC power is carried to the loads (i.e., the terminals) over the twisted pairs of Category 5 cabling that are not needed for Ethernet data communications, or over the data-carrying pairs, depending on the system configuration, as specified in the 802.3af draft standard. The power sourcing equipment in the hub is commonly referred to as Power Sourcing Equipment (PSE), while each terminal that receives the power is referred to as a Powered Device (PD). The PSE may be integrated with the switch, in what is known as an “end-span” configuration, or it may alternatively be located between the switch and the terminals, in a “mid-span” configuration.
A LAN in which a PSE is operating may include not only PDs, but also legacy terminals that are not configured to receive power over the LAN. In order to avoid damaging legacy equipment by applying high DC voltage to their LAN connections, the PSE must be able to determine, for each of its power output ports, whether or not the output is connected to a PD. For this purpose, the IEEE 802.3af Draft requires that each PD include a “signature element”—a special circuit across the power input connections of the PD, with predefined impedance characteristics. When the PSE is powered up, or when a new terminal is added to the LAN, the PSE performs a line interrogation routine in order to detect the signature element. During the line interrogation phase, the remaining circuits of the PD (other than the signature element) are isolated from the line by a switch. Upon successful completion of the interrogation, the isolating switch is closed, and the PSE begins to supply power to the PD. The interrogation routine uses low-voltage signals, in order to avoid damaging legacy terminal equipment.
Once the PSE has begun to supply power to a PD, it must also be able to detect when the PD is disconnected from the LAN, in order to avoid leaving high DC voltage on the open line. For this purpose, the IEEE 802.3af Draft specifies that the PSE should continuously sense the DC current that it supplies to the PD. If the current drawn from a given output port of the PSE drops below a predetermined threshold for a certain period of time, the PSE shuts off its DC output voltage to that port. This disconnect detection mechanism solves the problem of leaving DC voltage on an open line, and it prevents equipment damage in the event that a legacy terminal is connected in place of the disconnected PD. The mechanism requires, however, that the PD consume a certain amount of current at all times, even when it is idle. Otherwise, the PSE will cut off power to the PD. There is no simple way under the current standard to automatically restore the power after it is cut off.
PCT Patent Publication WO 99/53627, whose disclosure is incorporated herein by reference, describes a system for generating and monitoring data over pre-existing conductors of a network. One of the purposes of this system is to detect when a piece of equipment is removed from the network. A central module on the network includes an isolation power supply, which applies continuous DC power to the data lines of the network. The central module may modulate this DC power at low frequency in order to convey status information to remote modules connected to the data lines. The remote modules may use the power supplied by the central module to return modulated low-frequency current signals to the central module. A receiver in the central module monitors the low-frequency current data on the data lines and is thus able to detect removal and relocation of the remote modules.